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As a general rule for all ground covers, even grass, you don't want people walking on it, especially if there's going to be a lot of traffic. Walking causes soil compaction, depriving ground covers of air and water resulting in dirt pathways. If you want people to use your green room look at a system of roof pavers.

When installing roof pavers and intensive green roofs you're pretty much into building a structural bathtub for the pavers and plants to sit in. I worked on an intensive roof yerars ago and we used 13 in. offset from our interior finish floor to the exterior structural deck.

Attached are some photos of an intensive roof garden done by Reich & Associates. You can see at the base of the wood arbor columns the conc. pedestals for the structure, the roofing membrane and insulation board before the pavers are finally installed.

You will also notice that they did the modular pre-vegetated roof system where the plants come in trays with soil ready to go. A drip irrigation system is hooked up to all the modules so that when you need water, which we do even with over 60 in. per year, the plants don't die.

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Finally, you don't need a lot of soil to plant real trees. You'll just be limited in their height. I've done some in about 4 to 5 foot of soil. The trees came in wood planter boxes and we left them in them. The fill around the trees was mostly lightweight cinders, similar to the expanded material they make concrete blocks out of but about the 1/8" or less in dia. It's extremely lightweight, hold water really well, and plant roots will grow into it, thought it has no nutrients.

I see if I can post photos of the intensive design I did but you can create small hills up to and around tress and large shrubs so not all the roof surface doesn't have to be extremely deep.

For plant selection find an expert. Roof gardens are extreme enviroments, not only from a water issue but wind velocities can get extremely high. I had one small tree stripped of it's leaves only on the building side when the wind came down off a 12 story office tower. Looked bad especially since it was the focal point of the landscape design.

Also, roof gardens don't have the advantage of being heated by the ground. So elevated roofs not over heated areas can get super cold similar to early freezing of highway bridges in the winter. That's why you install insulation board under the plants, the roots are at most risk to freezing.

Hope that helps and I'll search for the old photos. I have one of a tree growing in just 6" of soil (it was added after we finished the project).

Comment

As a general rule for all ground covers, even grass, you don't want people walking on it, especially if there's going to be a lot of traffic. Walking causes soil compaction, depriving ground covers of air and water resulting in dirt pathways.

Did you use soil or soil supplements in combination with soil to retain moisture and reduce dead load? Most of the green roof people I have talked to recently seem to be heavily supplementing for these reasons. There is even a product out (produced in Japan, but damned if I can find its name at the moment) that is ceramic coated activated charcoal beads. Light weight, maintains and releases water slowly, and filters at the same time. I'll try to find its name.

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Rouser,
I did use a soil supplement but only as a light weight fill. I had a local university professor consult on the soil mixture and was fortunate to have have the availability of a local soil that didn't need any other supplements. Making sure your medium can retain sufficient moisture is critical. Back in the 70's some design used layers of newpaper but there are better options now.

The 120 mm should be sufficient for a mild climate. Some design use the insulation board as filler in lieu of additional soils because of cost. One of the problems I had was that during a heavy rain, water got between the roofing membrane and the insulation and caused the pavers to "float" up. During such downpours no one is on the roof garden so it's not a problem but a lot of water was a problem.

Comment

Rouser,
I did use a soil supplement but only as a light weight fill. I had a local university professor consult on the soil mixture and was fortunate to have have the availability of a local soil that didn't need any other supplements. Making sure your medium can retain sufficient moisture is critical. Back in the 70's some design used layers of newpaper but there are better options now.

The 120 mm should be sufficient for a mild climate. Some design use the insulation board as filler in lieu of additional soils because of cost. One of the problems I had was that during a heavy rain, water got between the roofing membrane and the insulation and caused the pavers to "float" up. During such downpours no one is on the roof garden so it's not a problem but a lot of water was a problem.

Comment

The three trees between the canopy and the building are replacement from the original design. I you look close you'll notice that the center one is located in a raised bed, approximately 4 ft. total depth. The other two were planted in 6" of soil. This variety has no deep roots, which is why it survived.

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The bottom photo show two of the original trees in the raised planters. Both have grown to between 20 and 25 feet high, in four feet of soil. Notice that under the tree in front of them there is no ground cover as it has died off and never been replaced. Additionally, the maintenance has been non-existent the last several decades and the drip irrigation system isn't working. All the plant materials are living strictly off of rainfall.